A blastocoel(e) or blastocele (also called blastocyst cavity, cleavage cavity or segmentation cavity) is the fluid-filled central region of a blastula and mammalian blastocyst. A blastocoele forms during embryogenesis when a zygote (a fertilized ovum) divides into many cells through mitosis.The adjectival of "blastocoel(e)" is blastocoelic.A blastocoel can be described as the first cell cavity formed as the embryo enlarges. It is essential for later gastrulation.